I'll preface this with [url]TFwiki.net[/url] is your best friend.
Takara is the company that provides to Japan only. Hasbro provides for the rest of the world. Molds are created as a joint venture between the two companies.
United, Henkei! Henkei!, and Gentei! Gentei! are Takara's "classics" toylines.
Classics, Universe, and Generations are Hasbro's "classics" toylines.
People often use the term "Chug" for short, meaning Classics, Henkei, Universe, Generations/Gentei, to refer to any release of a "classics" character under one of those lines. There are tons of lines, but you'll memorize all of the shorthand before long.
Diaclone - The Pre-transformers toyline which largely became Transformers
G1 - Generation 1 toys from the 80's
G2 - Gen 2 toys from the early 90's
BW - Beast Wars
BM - Beast Machines
RiD - Robots in Disguise , called Car Robots in Japan
UT - Unicron Trilogy, consisting of Armada, Energon, and Cybertron
AEC - see above
ML - Micron Legends, Armada in Japan
SL - Super Link, Energon in Japan
GF - Galaxy Force, Cybertron in Japan
Universe/Universe 1.0 - The 2003 Universe line, which started the trend of repainting existing molds as older characters
Alternators/Binaltech - A toyline and fiction in which all of the altmodes are licensed scale models
Movie - anything from the '07 Michael Bay movie line
TFA - Transformers Animated
Universe 2.0 - the 2008 Universe line, which continued the trend from the first Universe line
Rotf - Revenge of the Fallen
Rts and Hftd - Reveal the Shield and Hunt for the Decepticons imprint lines from the Rotf line
Prid - Prime Robots in Disguise, the current line for the Transformers Prime cartoon
dotm - Dark of the moon
Check out the link for more info.
http://tfwiki.net/wiki/Transformers:_Universe_%282003_toyline%29